1. Field of the Invention
This invention relates to an internal, transpiring-wall, platelet heat source and method of use in a supercritical water oxidation reactor and, in particular, to such a method and apparatus useful for the destruction of organic waste materials.
2. Description of the Related Art
It is known that the fluid and solvating properties of water change dramatically at its thermodynamic "critical point"--i.e., at a temperature of about 706.degree. F. and a pressure of about 3204 pounds per square inch ("psi"). In particular, water above its critical point is a single-phase fluid which is completely miscible with oxygen and most organic compounds. At supercritical water conditions, mass transfer limitations which limit the usefulness of oxidation processes in subcritical water are eliminated and the solubility of inorganic salts drops to the parts per million range. With the addition of oxygen, efficient destruction of organic compounds by oxidation is achieved, while inorganic substances can be separated and withdrawn for disposal. Specifically, organic compounds may be oxidized in supercritical water to produce carbon dioxide and water. Following the oxidation, inorganic materials may be removed as dry solids or precipitated as salts and removed as a brine. A significant advantage of such a process is the extremely short residence time required, on the order of 5-20 seconds depending on process temperature, for destruction of organic compounds.
Supercritical water oxidation--defined as a process for oxidizing organic waste compounds in supercritical water--has emerged as a potentially environmentally attractive technique for the safe and effective treatment of organic wastes and sludges. However, while organic compounds are completely miscible in supercritical water, inorganic salts present in the aqueous waste materials, or generated during the oxidation reaction, are essentially insoluble and may be deposited as solids on reactor and other system surfaces, which may lead to plugging and low operating efficiencies. In addition to that solids deposition problem, corrosion of reactor and system surfaces has also occurred in supercritical water oxidation processes. Due to these problems, supercritical water oxidation generally has not been successfully commercialized for the treatment of organic wastes and sludges.
Various attempts have been made to rectify these problems. For instance, U.S. Pat. No. 4,822,497 discloses a reactor designed so that the supercritical temperature process stream is transferred to a cooler zone in the same vessel at high pressure to form a brine which facilitates the removal of solids. More recently, U.S. Pat. No. 5,252,224 discloses a supercritical water oxidation process in which the reaction mixture is passed through the reactor at a velocity sufficient to prevent settling of a substantial portion of the solid particles from the reaction mixture.
There is still a need for a supercritical water oxidation process and system which avoids the above-mentioned significant problems of solids deposition and corrosion.
Still more recently, commonly assigned, copending U.S. patent application Ser. No. 08/320,426, filed Oct. 14, 1994, has proposed solutions to the foregoing problems. FIG. 1 illustrates one embodiment of a process and apparatus for the supercritical water oxidation of an organic waste material according to that copending application.
As shown in FIG. 1, a waste material at a high pressure (e.g., about 3500-6000 psi) is fed with oxidant via line 16 to a tubular reactor 17. The temperature of the waste is typically ambient (e.g., about 70.degree. F.) but may be higher. Depending on the waste material, the temperature should be no more than about 650.degree. F.
Oxygen or other suitable oxidizing agent is supplied with the waste material to the reactor 17 via line 16. Alternatively, or in addition, oxygen (or other oxidant) may be supplied to one or more points along the length of the reactor. For example, as shown in FIG. 1, oxygen may be supplied to reactor 17 through lines 45 and 42. The place of oxidant addition to the reactor depends on the properties of the particular waste being treated. The addition of oxygen along the length of the reactor permits control of the reaction rate, and hence the temperature rise, within the reactor.
If the intrinsic heat value of the waste material is low, a supplemental fuel also can be fed to the reactor along with the pressurized stream of waste material and oxidant. Any convenient gaseous or liquid fuel may be employed (e.g., alcohol, methane, methanol, etc.). Ignition of the supplemental fuel in the reactor provides the additional energy necessary to sustain the oxidation reaction. Preferably, however, the temperature of the pressurized waste/oxidant stream is increased from about ambient to reaction temperature by injection of water at supercritical conditions into the waste stream at one or more stages along the tubular reactor.
Reactor 17 generally comprises a platelet tube concentrically supported within a tubular shell. By "platelet tube" is meant a tube whose wall is formed of a plurality of stacked, thin plates having a large number of precisely engineered fluid passages formed therein to allow a fluid such as water to pass through the wall from outside to inside, the number and shape of the interior openings of those fluid passages being designed to provide a thin, protective film of water on substantially the entire interior platelet tube surface.
Platelet tubes are known to be useful, for example, in cooling surfaces of aerospace vehicles. They are commercially available from Aerojet-General Corporation, Rancho Cordova, Calif.
Generally, platelet tubes may be designed so that the injected supercritical water heats the waste stream to reaction temperature and provides protection to the inner reactor surface. In order to heat the waste stream, the platelet wall fluid passages may be designed to inject relatively large streams of supercritical water directly into the main body of the waste stream at appropriate points along the length of the reactor. In order to protect the inner surface of the platelet tube from solids deposition and corrosion, the platelet wall may also include a large number of smaller openings for injecting supercritical water into the region along that wall. The particular design of such smaller openings is not critical and may be varied as long as an adequate protective thickness of clean, supercritical water is provided on substantially the entire inner surface of the platelet tube. For example, a "transpiration" type platelet tube may be employed, in which a large number of small openings are provided in the plate forming the inner surface of the tube such that fluid is injected essentially in a direction perpendicular to the tube inner surface. The net result of this type of fluid injection is that the injected fluid will mix with the main flow of waste and simultaneously provide a thin film of clean supercritical fluid along the reactor wall.
Alternatively, and in a preferred embodiment according to the aforementioned copending application, the small surface openings are specially designed to inject the fluid in a direction substantially parallel to the interior tube surface, laying down on that surface a thin protective film of supercritical fluid. In this embodiment, that protective film will be provided for a short length along the interior tube wall from each aperture, after which the fluid will tend to mix with the main body of waste and increase its temperature. In another embodiment, both types of small openings (i.e., perpendicular and parallel injection) may be provided in the platelet tube inner surface to simultaneously heat the waste stream and protect the platelet tube inner surface.
FIG. 2 schematically illustrates how the supercritical water may be supplied to the central passage of the platelet tube to both heat the waste stream and form a protective film along the inner surface of the platelet tube. As shown, waste is fed to and along the central passage of the platelet tube and supercritical water is fed to the annulus between the platelet tube and the shell. Fluid passages in the platelet tube wall, and a pressure drop across that wall, permit supercritical water to flow from the annulus through the tube wall and then into the central passage of the tube. The direction and amount of flow of supercritical water into the central passage is controlled by the size and shape of the fluid passages in the tube wall. For example, in order to heat the waste in the central passage, a number of generally perpendicular streams (designated "A" in FIG. 2) may be provided, whereas a large number of generally parallel streams (designated "B" in FIG. 2) provide wall protection.
An example of a fluid flow pattern through the platelet tube wall that efficiently covers its inside surface with supercritical water is illustrated in FIG. 3. That figure shows an exploded view of a portion of a platelet tube wall composed of seven different plates 100 through 106. The pattern of flow in the illustrated embodiment is essentially hexagonal, with a single fluid inlet stream 107 being dispersed into 1,134 flow paths at the opposite side of the platelet wall. Specifically, a single fluid inlet stream 107 enters the tube wall through hole 108 in plate 100 and is immediately evenly distributed by six channels in plate 101 to six outlets on the hexagon in plate 102 (only four of which, reference numerals 109 through 112, are shown in FIG. 3) as well as flowing straight through hole 113 in plate 102. The stream in each of those outlets is then split into six other streams by hexagonal channels in plate 103, and a stream which flows straight through. The process is repeated through plates 104, 105 and 106 until the fluid finally exits the wall as 1,134 separate streams schematically illustrated 114 in FIG. 3.
The shape of the apertures in plate 106 determines the direction of the fluid stream exiting those apertures into the central portion of the platelet tube. For example, FIGS. 4 and 5 illustrate an embodiment which creates a flow of fluid in a direction generally parallel to the flow of waste and along inner surface 115 of the platelet tube. FIG. 4 (which is a plan view of a portion of the inside surface of the platelet tube) shows a plurality of apertures 116 in plate 106, each aperture being defined by a circular opening 117 partially overlaid by a trapezoidal opening 118. As shown best in FIG. 5 (which is a cross-section taken along line 6--6 of FIG. 4), fluid flowing out through circular opening 117 is deflected by surface 119 so as to flow along inside tube surface 115. The apertures may be formed by any convenient technique.
FIG. 6 illustrates an alternative embodiment in which the many apertures in the inner surface of the platelet tube are shaped such that the fluid streams emanating from them are directed substantially perpendicular to the inner surface of the tube, as shown schematically in FIG. 3. However, in addition to mixing with the main body of the waste stream to heat it, these many streams of clean, supercritical water also flow along the interior wall of the platelet tube to form a thin protective film.
FIG. 7 illustrates yet another embodiment in which a combination of transpiration apertures 120 and wall protection apertures 116 are provided in the inner surface of the platelet tube. With such a design, and by appropriate selection of the number and size of each type of aperture, the degree of wall protection can be controlled along with the effect on the temperature of the waste stream in the reactor by the injected supercritical water.
As illustrated in FIG. 1, reactor 17 comprises a platelet tube 50 concentrically supported within a tubular shell 51 and support members 52. The platelet tube 50 defines a center tubular passage 60 into which the waste/oxidant mixture is fed for reaction. Between the outer wall surface of platelet tube 50 and the inner wall surface of shell 51 is provided an annular space 53 into which supercritical water is injected at one or more locations along the length of the reaction zone.
Because of the differential between the relatively high pressure of the injected supercritical water in annular space 53 and the somewhat lower pressure of the waste stream in center passage 60, the supercritical water readily flows from the annular space 53 through the fluid passages provided in the wall of the platelet tube 50, into center passage 60. As noted above, the fluid passage apertures provided on the inner surface of platelet tube 50 are designed to disperse the injected supercritical water as a thin film along that inner surface. As a result, the temperature of the waste/oxidant mixture in central passage 60 is increased while the thin film of water along the inner surface of platelet tube 50 protects that surface against deposition of solid material and corrosion. The thickness of the protective film is not critical. It is only necessary that the supercritical water be provided to the platelet tube so that a film is formed on the inner surface thereof so as to form a substantially continuous isolating layer of pure fluid between the reactor wall and the waste stream undergoing reaction.
The number and location of points along the reactor at which supercritical water is introduced into the annulus 53 are not critical and depend on, for example, the temperature, flow rate and heat content of the waste stream, the temperature and flow rate of the supercritical water, the size of the reactor, etc. The four points of addition shown in FIG. 1 are merely illustrative, and those skilled in the art recognize that a single addition may be sufficient, or that more than four points of addition can be used. Generally, the water injection along the length of the reactor is varied to allow temperature control within the reactor, provide an even flow distribution of water along the length of the platelet tube, assure substantially complete coverage of the surface of the reaction zone by a protective film of clean supercritical water, etc.
A substantial advantage of the design described in the aforementioned copending application is the avoidance of solids deposition and corrosion on the reactor walls. This is accomplished by providing the thin, protective film of water on the interior surface of the reactor tube, as described above, and also by eliminating extensive preheating of the waste material prior to reaction, which can cause both solids deposition and corrosion. Instead, according to this design, the waste is heated to reaction temperature by the injected supercritical water which may itself be heated outside the reactor by any suitable means. As shown in FIG. 1, for example, boiler-quality (i.e., clean) water, which is heated, is fed to the reaction zone through lines 40, 41, 42 and 43. In an alternative embodiment, pressurized colder water (e.g., about 4000 psi or higher and about 300.degree. F.) may be injected into the reaction zone at or near the end of the reactor to quench the reaction products. In FIG. 1, this embodiment is illustrated by stream 44.
The particular oxidant employed for the reaction is not critical, and gaseous oxygen is preferred. Other oxidants which may be employed include hydrogen peroxide, air, etc.
FIG. 1 also shows a method of cooling and separating the reaction product components. As shown, the reactor 17 discharges directly into a liquid/gas separator 18 through inlet 55, and from which gaseous products are vented via stream 30. The solid and liquid components of the reaction mixture are cooled in separator 18 to a temperature of 200.degree.-600.degree. F. by means of a cooled recycle stream. According to the aforementioned copending application, it is desirable, and therefore preferred, to cool the reaction products as quickly as possible to re-solubilize the solid inorganics as a brine. This may be facilitated by supplying pressurized colder water at or near the end of the reactor, as described above, to quench the reaction products. The temperature of the quench water is not critical and may be at any temperature below supercritical, for example, about 300.degree. F. The resulting cooled liquid/solid mixture is removed from separator 18 through outlet 56 via stream 19, at which point it is split into two streams 20 and 25. Stream 25 may be chemically treated, if desired, to neutralize any acids present by addition of an appropriate chemical (e.g., sodium hydroxide, etc.) through line 27. The resulting neutralized material preferably has a pH of about 7. The resulting stream 28 can then be cooled in a heat exchanger (not shown) to a temperature below the supercritical temperature and recycled to separator 18 via line 29. It is preferred to tangentially introduce the recycle stream 29 into separator 18 above a channel separator 59 to provide a wetted wall and avoid salt accumulation.
Stream 20 can be cooled in the heat exchanger to facilitate disposal or further treatment.
As mentioned above, the process and apparatus described in the aforementioned copending application may involve the use of a combustion device located at the reactor inlet to bring the incoming waste/oxidant stream up to reaction temperature. For example, a suitable combustor or burner may be located at the inlet of the platelet tube reactor. A suitable fuel (e.g., alcohol, methane, methanol or similar liquid or gaseous fuel) and oxygen (or other oxidant) are separately preheated externally of the reactor and fed to the burner at the reactor entrance. Ignition of the fuel occurs at the burner face and increases the temperature of the incoming waste stream. The type and amount of fuel, and the amount of oxygen, and their respective flow rates and temperatures may be varied to control the temperature increase experienced by the waste at the reactor inlet.
FIG. 8 schematically illustrates a burner located at the inlet of the platelet tube. As shown in FIG. 8, the fuel and oxygen fed to the burner 200 are mixed in the area of a cone-shaped diffuser 201, surrounded by the waste stream. When the waste stream flows over the diffuser, a low pressure center is created at the diffuser face which induces circulation and mixing of the fuel and oxygen, thereby providing an efficient ignition and heating of the waste material. Liquid and gaseous fuels of any kind can be employed. Ignition of the fuel at the diffuser face of the burner is assured by preheating both the fuel and oxygen streams, using any suitable technique.
The concept of preheating internally as described is also of particular interest in those cases where the waste or hydrocarbon containing slurry is treated at supercritical water conditions to effect separation of organic and inorganic materials. Oxidation of the separated organic materials may be performed outside of the reactor for purposes of generating power, etc. The internal preheater in the case described above serves the purpose of bringing the stream to be treated to the critical temperature and above to effect the separation desired. For example, a platelet tube reactor system may be employed to separate organics from a waste stream under supercritical conditions for subsequent oxidation, combustion, power generation, etc.
The foregoing design provides significant advantages by avoiding the solids deposition and corrosion associated with conventional supercritical water oxidation reactors.
The foregoing design overcomes other drawbacks, as well. Generally speaking, there is a concern that very large amounts of supercritical water (on the order of 1100.degree. F.) will be needed to preheat the sludge/water mixture to the process temperature of about 1050.degree. F. It is known that the temperature of the heating water cannot be increased much over 1100.degree. F. due to the material strength of the vessel at that temperature and at the operating pressure of about 4000-4500 psi in the vessel. One solution to this, discussed above with respect to FIG. 8, is to use a submerged combustor in the center of the platelet to preheat the sludge/water mixture to the process temperature.
However, there is a need for alternative designs to preheat the sludge/water mixture to the process temperature, while at the same time providing those advantages of the foregoing design.